The present invention relates to a method and apparatus for accomplishing in situ vitrification of soil and waste materials and particularly to such a method and apparatus as promotes the formation and maintenance of a useful cold cap layer.
The presence of waste or contaminated material buried in the ground is a subject of concern relative to possible dispersion of the waste and contamination of the surrounding environment. Various methods have been employed for stabilizing or solidifying waste sites, for example, the injection of soil reactant chemicals or solidifying compositions of material such as concrete. Stabilization has also been accomplished by artificially freezing the soil.
A particularly attractive method for solidifying or destroying hazardous waste material in situ and for general soil stabilization is described and claimed in Brouns et al U.S. Pat. No. 4,376,598 entitled "In Situ Vitrification of Soil", issued Mar. 15, 1983. According to this method, two or more electrodes are placed in the ground and after a start-up procedure a substantial current is passed therebetween resulting in liquification or melting of the soil. The waste materials will be melted, pyrolyzed or dissolved in the molten soil. A gas collection apparatus is placed over the site for removing gaseous products of combustion. After melting, the current flow is discontinued whereupon the liquefied mass hardens into vitrified material comprising for the most part a glass, a glass ceramic or a partially devitrified glass, having crystals and glass dispersed in a solid matrix composed primarily of silica and alumina materials. Any metals within the mass may be dissolved in the glass especially as oxides, or may form accumulations at its lower boundaries.
According to this procedure, the soil undergoing treatment reduces somewhat in bulk and "subsides" leaving a depressed area or pocket which may then be backfilled to the original ground surface further burying the solidified waste material. This mode of operation is achieved, for example, when employing electrodes exhibiting high electrical and heat conductivity such as graphite electrodes which promote subsidence of the soil surface.
However, subsidence of the surface is not always desirable since it may allow the ready escape of volatile and/or hazardous materials as may be contained in organic waste materials.
It is therefore an object of the present invention to increase the residence time of volatile and/or hazardous materials in and around the electrodes and the liquefied material in an in-situ vitrification process in order to promote their complete destruction by pyrolysis.
It is another object of the present invention to employ the natural materials available in forming a structure useful in promoting the destruction of organic type volatile and/or hazardous materials during in situ vitrification of soil and waste materials.
It is a further object of the present invention to promote the destruction of organic type volatile and/or hazardous materials without fundamentally altering the basic steps for the in situ vitrification of waste materials.